1. Field of the Invention
The present invention relates to a water electrolysis system having a water electrolysis apparatus for electrolyzing water with an electric current supplied from a DC power supply to generate hydrogen and oxygen and a high-pressure hydrogen processing apparatus disposed downstream of and connected to a hydrogen outlet of the water electrolysis apparatus, for refining high-pressure hydrogen discharged from the hydrogen outlet which discharges hydrogen under a pressure higher than a normal pressure, and a method of shutting down such a water electrolysis system.
2. Description of the Related Art
Solid polymer electrolyte fuel cells generate DC electric energy when anodes thereof are supplied with a fuel gas, i.e., a gas mainly composed of hydrogen, e.g., a hydrogen gas, and cathodes thereof are supplied with an oxygen-containing gas, a gas mainly composed of oxygen, e.g., air.
Generally, water electrolysis apparatus are used to generate a hydrogen gas for use as a fuel gas for such solid polymer electrolyte fuel cell. The water electrolysis apparatus employ a solid polymer electrolyte membrane (ion exchange membrane) for decomposing water to generate hydrogen (and oxygen). Electrode catalyst layers are disposed on the respective sides of the solid polymer electrolyte membrane, making up a membrane electrode assembly. Electric feeders are disposed on the respective sides of the membrane electrode assembly, making up a unit. The unit is essentially similar in structure to the fuel cells described above.
A plurality of such units are stacked, and a voltage is applied across the stack while water is supplied to the electric feeders on the anode side. On the anodes of the membrane electrode assembly, the water is decomposed to produce hydrogen ions (protons). The hydrogen ions move through the solid polymer electrolyte membranes to the cathodes, where the hydrogen ions combine with electrons to generate hydrogen. On the anodes, oxygen generated together with hydrogen is discharged with excess water from the units.
Such a water electrolysis system generates hydrogen under a high pressure of several tens MPa. Japanese Laid-Open Patent Publication No. 2007-100204 discloses a method of and an apparatus for manufacturing high-pressure hydrogen. As shown in FIG. 7 of the accompanying drawings, the disclosed apparatus for manufacturing high-pressure hydrogen comprises a high-pressure oxygen vessel 1, a differential pressure regulator 2, a high-pressure hydrogen vessel 3, an electrolytic cell 4, a moisture-adsorption tube 5, a back-pressure valve 6, and a deoxidization tube 7.
Pure water contained in the high-pressure oxygen vessel 1 is delivered to an anode side of the electrolytic cell 4 by a circulation pump 8. The pure water delivered to the electrolytic cell 4 is electrolyzed when the electrolytic cell 4 is energized by a power supply 9. Oxygen generated from the pure water by the electrolytic cell 4 is delivered, together with returning pure water from the circulation pump 8, to the high-pressure oxygen vessel 1.
Hydrogen generated at the cathode of the electrolytic cell 4 is discharged, together with permeated water, into the high-pressure hydrogen vessel 3. At this time, the differential pressure regulator 2 equalizes the pressure in the high-pressure oxygen vessel 1 and the pressure in the high-pressure hydrogen vessel 3 to each other.
The hydrogen stored in the high-pressure hydrogen vessel 3 is delivered to the deoxidization tube 7, which removes oxygen contained in the hydrogen. The hydrogen is then delivered from the deoxidization tube 7 through the back-pressure valve 6 to the moisture-adsorption tube 5, which removes moisture from the hydrogen, thereby finalizing the hydrogen as a product.
As described above, the apparatus for manufacturing high-pressure hydrogen disclosed in Japanese Laid-Open Patent Publication No. 2007-100204 generates high-pressure hydrogen and high-pressure oxygen, using the high-pressure oxygen vessel 1 and the high-pressure hydrogen vessel 3. Another water electrolysis system for generating high-pressure hydrogen employs a differential pressure generating process which keeps generated hydrogen under a high pressure while keeping water and generated oxygen under a normal pressure.
According to the differential pressure generating process, when the water electrolysis system is shut down, it is necessary to return a high-pressure hydrogen line in the water electrolysis apparatus (electrolytic cell) back to the normal pressure (atmospheric pressure). This is because if the solid polymer electrolyte membrane is maintained under a differential pressure at all times, then the solid polymer electrolyte membrane is susceptible to damage.
Practically, the pressure of the high-pressure hydrogen which remains in a facility that is connected downstream of the water electrolysis apparatus, e.g., the deoxidization tube 7 or the moisture-adsorption tube 5 in Japanese Laid-Open Patent Publication No. 2007-100204, is reduced to the normal pressure. Since the high-pressure hydrogen remaining in the high-pressure line is depressurized, the depressurized high-pressure hydrogen is wasted. Therefore, the water electrolysis system is uneconomical.
In addition, when the water electrolysis system is to be resumed, it takes a considerable time to pressurize the high-pressure hydrogen line downstream of the water electrolysis apparatus, i.e., to fill the high-pressure hydrogen line downstream of the water electrolysis apparatus with high-pressure hydrogen. Consequently, the resumption of the water electrolysis process requires an increase in the power consumption, is time-consuming and hence is not efficient.